1. Field of the Invention
The present invention relates to a process for the reduction of nitrogen oxides from exhaust gases by their reaction with reducing agents on catalysts.
2. Background Information
The selective removal of nitrogen oxides from exhaust gases is generally carried out by a reaction with ammonia. In the absence of catalysts, this reaction proceeds at temperatures in the region of 900.degree. C. in the gaseous phase. This temperature may be reduced by means of special catalysts. It is desirable to lower the temperature as much as possible in order to avoid the expensive heating up of flue gases, in particular after a desulphurization of the flue gas. A technical catalyst is required not only to be capable of high catalyst loading with as far as possible complete conversion, but also to have a long catalyst life with low pressure losses.
In the widely used SCR process (selective catalytic reduction) using catalysts based on TiO.sub.2, it is necessary to employ temperatures above 350.degree. C., which partly necessitates the uneconomical reheating of the gases. Moreover, catalysts suitable for such processes (DE-C 2 458 888=U.S. 4,085,193) are expensive to produce.
The catalytic degradation of NO.sub.x with ammonia from active charcoal proceeding at temperatures of from 120.degree. to 150.degree. C. has been disclosed in DE-A 2 911 712. One disadvantage of this process is that it requires very large quantities of coal. The amount of active coke required for a power plant conducting 300 MW is about 50 tons per year with an initial filling of about 500 t.